The easy, short answer: around 14 to 16 meters.
aircrafts at that weight range seem to have wingspans between 14 and 16 meters. Your animal will need bigger wings due to the higher gravity though so you might want to increase that number a bit. This answer ignores all matters such as the constraints of a flying creature.
The long, hard answer: within the constraints of earth biology your mammalian dragon isn't going to leave the ground anytime soon.
The picture right below depicts an extinct creature known as Hatzegopteryx.
This lovely pterosaur is, without a doubt, my favorite of all and the best example of a real life dragon. Why exactly? Because as far as we are aware, it could grow to be up to 6 meters tall (roughly the average height of a giraffe) and was surprisingly sturdy when compared to its relatives, such as quetzalcoatlus. They needed this added bulk because, as far as we know, they are one of the few, if not the only, group of azhdarchid pterosaurs to ever play the role of apex predator in its habitat, hazteg Island, due to most of the creatures in the island suffering from insular dwarfism and the lack of competition from larger theropods. Since it's overall very similar to its other family members and quetzalcoatlus' maximum flying speed is estimated to be closer to 128 km/h, it's believable that hatzegopteryx could reach similar speeds.
Essentially, hatzegopteryx was big, could fly, was relatively sturdy for an azhdarchid pterosaur and was an apex predator. Definitely high up on the list of creatures closest to functioning like an actual dragon without having to play the card of remaining small (almost every record when proportionality is taken into account belongs to arthropods fro a reason. Being big brings a lot of limitations).
Now, the important information: to accomplish its purpose as a flying apex predator in its turf, estimates of hatzegopteryx had a wingspan of around 10 to 12 meters, and a maximum weight estimate of...250 kg give or take. Any heavier and it could give goodbye to flying.
You see, azhdarchid pterosaurs like hatzegopteryx didn't just get their weird appearance for no reason. The ability to fly demands a lot, and the bigger you are, the heavier you get, but the heavier you get, the harder it is to fly.
Normally the approach to solve this would be to add more power to the wing flap, but more power means more muscle, which means more weight.
Azhdarchid pterosaurs got their looks due to being extremely well adapted for being Able to fly despite their large size. So well adapted in fact that no other group of animals ever managed to get close to growing nearly as big without sacrificing flight ability. These pterosaurs made use of their forelimbs the most, including when it came to taking off, meaning they could afford to have weaker hind limbs that didn't need as much muscle for them to sustain their weight, unlike birds who use their wings exclusively for flight and require powerful hindlimbs to walk on the ground. Pterosaurs also had a system of air sacks very similar to that of modern birds, meaning they could make the most out of every breath, ensuring they'd get the necessary oxigenation to fly. Their bones were also pneumatic, helping out with both the oxigenation matter and allowing them to be overall lighter than they'd need to be if their bones were like those of a mammal. The last important factor was their membranous wings, which, although more fragile than feathered wings, allowed for more energy efficiency in flight, as well as being better at Generating thrust and lift than a bird's.
Your mammalian dragon (and in here I'm assuming their traits are exactly like a normal mammal's) only has the membranous wings boom going for it. Their bones are much denser, which limits the maximum size they can reach before being too heavy to fly within the constrains of earth biology. Their respiratory system is by default less efficiently than an airsack system like what we see in birds, sauropods and pterosaurs (these 3 are among the main seasons why the biggest mammalian megafauna could never grow to be as big as the biggest sauropods). Furthermore, carrying around live young is no easy task, especially when you need to fly around (see how many bat species often don't have more than a single baby per pregnancy). All of these problems already affect growing big without loosing the ability to fly, and don't even cover problems such as the added weight of an extra pair of limbs, in the situation that your mammalian dragons are truly hexapodal and not quadrupedal.
The last problem with the situation is that your creatures don't live in a planet that makes it easy for fliers. Flying is essentially what we call swimming in the air, a fluid with density much, much lower than water. If you want to make flying easier and allow for bigger flying creatures, the key is to make it so that the air is denser and the pull of gravity weaker (aka a planet with gravity lower than that of earth and denser atmosphere). Your planet on the other hand has greater gravity, but atmosphere just as dense as those found on earth, that means that in your planet flying is even harder than in here.
So summing up things in a list:
the majority of anatomical qualities present in azhdarchids that allowed them to fly at such sizes (something that had already required a number of simply extreme adaptations to minimize weight and maximize efficiency with flight as the main selective pressure) are absent in your mammalian dragons
the weight of your mammalian dragons far exceeds the limits seen in the largest creatures to ever fly (Their lightest adult is around 8 times heavier)
they live in a planet that makes flight an even harder task than on earth, which means that within the constraints of biology as we know it, the maximum size anything that can fly could reach in your planet would be smaller than what could be found here.
Essentially, as of now your dragons will have to be happy about swimming on the water if they want something closer to the ability to fly, Because there are just too many things hindering them.
